A Systematic Procedure For Synthesizing Virtual Oscillators For Inverter-Based Power Systems

Publications:

Microgrids are a collection of energy sources interfaced to an ac electric distribution network that can be operated independently from the bulk ac system. Energy conversion is accomplished by power-electronics inverters, which are typically controlled to regulate the voltage amplitude and frequency of the inverters’ terminal voltage. Typically, the strategy employed to do this is droop control, which is only well defined in a sinusoidal steady-state and linearly trades off the inverter-voltage amplitude and frequency with real- and reactive-power output.

Description

NREL engineers have developed an alternative strategy to droop control to enable the design of scalable microgrids and systems containing large numbers of power electronics inverters. Virtual Oscillator Control (VOC) is a decentralized control strategy for ac systems where inverters are modulated to emulate the dynamics of weakly nonlinear oscillators. Compared to droop control, VOC is a time-domain controller that enables interconnected inverters to stabilize arbitrary initial conditions to a synchronized sinusoidal limit-cycle. Since nonlinear oscillators that are elemental to VOC cannot be designed with conventional linear-control design methods, NREL engineers have applied averaging- and perturbation-based nonlinear analysis methods to extract the sinusoidal steady-state and harmonic behavior of such oscillators. The averaged models reveal conclusive links between real- and reactive-power outputs and the terminal-voltage dynamics. Similarly, the perturbation methods aid in quantifying higher-order harmonics. The resultant models are then leveraged to formulate a design procedure for VOC such that the inverter satisfies standard ac performance specifications related to voltage regulation, frequency regulation, dynamic response, and harmonic content.

To enable deployment of VOC on digital controllers, NREL engineers have also invented a design methodology which allows a system designer to straightforwardly translate a set of ac power system performance specifications directly into control parameters. A coordinate transformation is also leveraged to enable the user to obtain tunable relationships between the real and reactive power delivered by the inverter and the system voltage and frequency. This design flexibility essentially allows VOC to subsume the functionality of traditional droop controllers while enabling enhanced speed and responsiveness to dynamic conditions.

Benefits

Synchronizes interconnected inverters to form an interconnected ac system

Facilitates the design of highly distributed systems with no communication between devices

Meets performance requirements of ac power systems

Ultra-fast response enables stabilization of volatile systems

Capable of working on controllers with different power, voltage, and current ratings